The radio frequency spectrum is used by a large number of radio broadcast and radio communication systems. In order for the different radio broadcast systems and radio communication systems to operate correctly, conflict between the signals used in the different radio broadcast systems and radio communication systems must be avoided. This is generally achieved by allocating different frequency bands of the radio frequency spectrum to each of the different radio broadcast systems and radio communication systems.
The demand for capacity in radio communication systems is high and is expected to increase in the future. However, the available radio frequency spectrum is not unlimited, and so it is desirable to enable different radio communication systems to operate in at least partially overlapping frequency bands in order to provide the desired increase in capacity in radio communication systems to meet the expected demand.
One example of an existing radio communication system is the Global System for Mobile communication (GSM) system. The GSM system was initially designed to carry mainly voice traffic, and is unable to provide high data transfer rates for data traffic. As a result the EDGE packet radio system (EGPRS) has been proposed as a development of the GSM system, to provide improved data capacity. The EDGE packet radio communications system (EGPRS) provides both a classic EGPRS system, using the same signal format as the current GSM system, and a COMPACT system which uses a different system format. It is envisaged that radio frequency spectrum used by the classic EGPRS system and by the COMPACT system will overlap with the radio frequency spectrum currently used for the GSM system. In the following, reference will be made to the GSM format signals: however it will be clear that this description applies to classic EGPRS system signals as well as to GSM signals.
Both the GSM and the COMPACT radio communication systems are cellular radio communication systems in which a network of base stations is provided. Each base station provides access to the GSM network or the COMPACT network for a number of mobile stations within a cell associated with that base station, by means of a radio frequency interface.
The frequency bands allocated to the GSM radio communication system and to the COMPACT radio communication system are divided into a number of radio frequency channels. As indicated above, the radio frequency channels are common to the two systems. Signals on the radio frequency channels are arranged as multi-frame signals comprising a fixed number of frames, each of the frames containing within it a number of time slots. A base station and a mobile station communicate by means of logical control channels and traffic channels which are mapped onto specified parts of the transmitted multi-frame signals.
A schematic diagram of the multi-frame format defined for GSM signals is shown in FIG. 1a. As can be seen, a multi-frame comprising 51 Time Division Multiple Access (TDMA) frames numbered 0-50 is used, each frame of the multi-frame being divided into eight time slots. A down link signal from the base station to the mobile station carrying the Broadcast Control Channel (BCCH) and Command Control Channel (CCCH) logic channels is shown, as an exemplary illustration of the multi-frame signal structure used in the GSM system. FIG. 1a also shows the presence of a Frequency Correction Channel (FCCH) containing a frequency correction burst (FCB) in timeslot 0 of frame nos. 0, 10, 20, 30 and 4, and the presence of a Synchronization Channel (SCH) containing a synchronization burst (SB) in timeslot 0 of frame nos. 1, 11, 21, 31 and 41.
A schematic diagram of the multi-frame format defined for COMPACT signals is shown in FIG. 1b. As can be seen, a multi-frame comprising 52 Time Division Multiple Access (TDMA) frames numbered 0-51 is used, each frame of the multi-frame being divided into eight time slots. A down link signal from the base station to the mobile station carrying the Compact Broadcast Channel Control (CPBCCH) and Compact Command Control Channel (CPCCCH) logic channels is shown, as an exemplary illustration of the multi-frame signal structure in the COMPACT system. FIG. 1b also shows the presence of a Frequency Correction Channel (CFCCH) containing a frequency correction burst (FCB) in timeslot 1 of frame no. 25, and the presence of a Synchronization Channel (CSCH) containing a synchronization burst (SB) in timeslot 1 of frame no. 51. It should be realized, however, that the frequency correction channel (CFCCH) and the synchronization channel (CSCH) in a multi-frame may be in any of corresponding ones of timeslots 1, 3, 5 or 7 of their respective frames.
For both the established GSM radio communication system and the new COMPACT radio communication system the frequency correction burst (FCB) is modulated to contain a tone and is provided to enable the mobile station to synchronize with a signal from the base station. The tone contained by the frequency correction burst (FCB) can easily be detected by the mobile station. If a frequency correction burst (FCB) is received, the mobile station can deduce that the currently received radio frequency channel also contains synchronization information, in the form of the synchronization burst (SB). The mobile station can then attempt to frame synchronize with a signal from the base station on the currently received radio frequency channel by attempting to locate the synchronization burst (SB) carried on the Synchronization Control Channel (CSCH).
However, the multi-frame structures used in the GSM radio communication system and the COMPACT radio communication system are different, as described above with reference to FIGS. 1a and 1b. As can be clearly seen from a comparison of FIGS. 1a and 1b, the synchronization burst (SB) in a GSM format multi-frame signal can be found at a position 10 or 11 frames after the position in the multi-frame of the frame containing the detected frequency correction burst (FCB): in contrast the synchronization burst (SB) in a COMPACT format multi-frame signal can be found at a position 26 frames after the position in the multi-frame of the frame containing the detected frequency correction burst (FCB).
In view of this, it has been proposed that the frequency correction burst used in the COMPACT system is modulated to contain a tone at −67.7 kHz offset from the radio frequency channel carrier, instead of a frequency correction burst (FCB) modulated to contain a tone at +67.7 kHz offset from the radio frequency channel carrier, as established for the GSM system. Since the tones carried by the COMPACT system frequency correction burst (FCB) and the GSM frequency correction burst (FCB) would be different, COMPACT mobile stations would be able to differentiate between a radio frequency channel carrying a GSM format multi-frame signal, and a radio frequency channel carrying a COMPACT format multi-frame signal.
However, a problem with this proposal is that a variety of different methods of detecting the frequency of the tone contained in the frequency correction burst (FCB) have been implemented in existing GSM compatible mobile stations in use today. One such method is to examine the power spectral density of a received signal around 67.7 kHz.
However, as is well known, the power spectral density for tones offset by ±f relative to a radio frequency carrier yield the same spectrum once translated into a baseband signal in the handset. This is because the signals are real and therefore have a power spectrum having an even symmetry relative to zero. This result is illustrated by FIG. 2, where FIG. 2a) shows the power spectral density S(ν) for a tone with a positive offset from the carrier and FIG. 2b) shows the power spectral density S(ν) for a tone with a negative offset from the carrier. The power spectral density Sb(ν) resulting from the down-converting of S(ν) is shown in FIG. 2c) and is seen to result from the contribution of both frequency mirror tones.
Therefore any existing GSM mobile station which detects the frequency of the tone contained in the frequency correction burst (FCB) using a power spectrum density evaluation method will be unable to distinguish between a frequency correction burst (FCB) tone at +67.7 kHz offset from the carrier and a frequency correction burst (FCB) tone at −67.7 kHz offset from the carrier. Any such existing GSM mobile station receiving a frequency correction burst (FCB) tone at −67.7 kHz offset from the carrier would attempt a synchronization burst decode operation at a position 10 or 11 frames after the position in the multi-frame of the frame containing the detected frequency correction burst (FCB). This synchronization burst decode operation would be unsuccessful for the reasons outlined above. Thus the time required for the GSM mobile station to synchronize with a GSM signal would undesirably be extended by the time taken for the failed attempt to synchronize with a received COMPACT signal.
In addition, the mobile station uses the difference between the frequency of the received tone and the expected frequency of +67.7 kHz to adjust the local oscillator. Thus, a GSM mobile station which detects the frequency of the tone contained in the frequency correction burst (FCB) using a power spectrum density evaluation method will make erroneous adjustments to its local oscillator on receipt of a −67.7 kHz tone.
Thus the present invention seeks to minimize the time necessary for a receiving radio device to establish synchronization with a signal received from a transmitting radio device capable of providing an appropriate service to the mobile station.